Draft Pressure Profile for Natural Draft Furnace

The <...pressure...> in the <...box...> is of less importance than minimising the excess oxygen passing through it. A <...standard practice...> is called "oxygen trim". The principle is to minimise the oxygen remaining in the exhaust gases by trimming the air passing through the furnace to obtain the most economical conditions for operation. Excess air carries heat away from the boiler instead of its being used to heat the boiler contents, so oxygen trim is generally seen as a Good Thing.

It is also of value in keeping the stack top temperature above the dew point of sulphuric acid so as to stop the thing rotting and bits falling off it to the ground. Excess air makes the stack temperature drop, and corrosion and early renewal more likely.

The original equipment manufacturer[OEM]'s Operation and Maintenance Manual may well have a section on this topic for reading, and the OEM will also be able to advise over the telephone. General principles can also be read in any text on the economics of boiler operation, on which topic there are dozens of published textbooks.

Google :

Boiler emissions reference guide.

A whole lot of downloadable pdf reference guides.

Something mechanical has happened to change the draft profile. Perhaps a damper that is supposed to be open is closed, your grate is plugged, or some other obstruction external to the boiler is affecting the flow of air to the furnace. It could be something as unlikely as an adjacent building draft change due to someone closing a garage door.

Assuming that your draft controller sensor is operational, if it is suddenly out of range to control the furnace draft, damper full open, for instance, perhaps the combustion air control is supplying less air than necessary to efficiently burn the fuel, starving the furnace area.

Also, be sure your instrumentation is correct, use another gauge to check...

The amount of fresh air required has many variables,including atmospheric variations.The ideal draft will depend on your specific load requirements.

The cheapest way is to analyze your stack gas for detection of CO2.There are many other chemicals in the stack gas,as well as corrosive chemicals produced,and Nitrous Oxide, but the main ones are CO2 and CO for determining efficiency.There are chemical as well as electronic methods of doing this,depending on your budget and operator skill level.

Basically, too much CO is a very rich(too little air) mixture,and too much CO2 is too much air.

Here is a link that explains things very simply:

https://www.pmmag.com/articles/100035-tips-for-combustion-analysis-of-commercial-boilers

Everything you wanted to know about combustion gases,but were afraid to ask.